CAM TIMMING AFECTS BY COMPETITION CAMS AND ME
I am not an expert . this is from info I have read and learned.
i have edited this artical .
Overlap
The amount of overlap, between the intake opening and the exhaust closing, and where it occurs, is one of the most critical points in the engine cycle.
This overlap phase, is very critical to vacuum, throttle response, and, gas mileage.
Remember this when u add bigger carbs. More overlap = less vacuum response. Smaller carbs require less vacuum response.
More overlap will lower cyl pressure at low rpms . Beneficial for preventing detonation.
This is why they recommend more compression with bigger cams, but this adds back the likelihood of detonation.
I think this is the most obvious cam characteristic. The more the overlap the higher the rpm the motor will breath at (it will move up the torque curve).
The exhaust stroke of the piston has pushed out just about all of the spent charge and as the piston approaches the top and the intake valve begins to open slowly, there begins a siphon or "scavenge" effect in the chamber. The rush of the gases out into the exhaust port will PULL in the start of the intake charge. This is how the engine flushes out all of the used charge. Even some of the new gases escape into the exhaust.
If the overlap phase occurs too early, it will create an overly rich condition in the exhaust port, severely hurting the gas mileage.
ADVANCING the intake and RETARDING the exhaust (“closing up the centers”) increases overlap and should move the power up in the RPM range, usually at the sacrifice of bottom end power. The result would be LOWER numerical values on both intake and exhaust lobe centers.
RETARDING the intake and ADVANCING the exhaust (“spreading the centers”) decreases overlap and should result in a wider
power band at the sacrifice of some top end power. This condition would be indicated by higher numerical values on both intake and exhaust lobe centers. By moving only one cam the results are less predictable, but usually it is the intake that is moved to change power characteristics since small changes here seem to have a greater effect.
the intake opening.
If the intake valve opens too early, it will push the new charge into the intake manifold. If it occurs too late, it will lean out the cylinder and hinder the performance.
the intake closing.
This occurs slightly after Bottom Dead Center.
The quicker it closes, the more cylinder pressure the engine will develop, creating more torque.
but, make sure that you hold the valve open long enough to properly fill the chamber,or u lose power.
*****The longer the intake valve stays open past bdc, u will get a inertia effect if the rpms are HIGH enough. As the incoming charge picks up speed it cant stop fast enough, and it will over stuff the cyl at HIGH rpm. Remember the piston almost stops at bdc as it changes direction. but it will LOOSE charge at low rpm. This is , due to the piston coming up and pushing the charge back out the intake, if the charge is moving tooo slow to create enough inertia to keep itself moving downward.
Therefore, delaying intake closing will move your power up or down the rpm curve.
The exhaust stroke.
Once the piston passes through Top Dead Center and starts back down, the intake charge is being pulled in quickly so the exhaust valve must close at precisely the right point after the top to keep any burnt gas from reentering.
The exhaust closing
If the exhaust valve closes too early it will trap some of the spent gases in the combustion chamber, diluting the next charge.
closes too late
if it closes too late it will over-scavenge the chamber; taking out too much of the charge, again creating an artificially lean condition
OK, Now what do we do with all this info??
Well for me, its just a tool.
As we know the motor is the sum of all the parts. Most importantly they must work together towards a common goal.
As we change parts we always compromise something.
As we add bigger carbs, header, bigger cams, we move the power up the rpm scale.
All of this means less bottom end.
Just a header or carb choice is a big change.
A header itself will cause reversion if not fitted to your other parts properly.
A large carb will move the power up dramatically at the expense of the bottom end response. This is where fuel injection saved the day.
As u can see the motor is affected in many ways.
I don’t expect changing the cam timing to be a cure for the wrong cam, it can only make a mild change.
If u feel a massive dif after degreeing , maybe u have some wrong components in the motor.
Every timing change has a compromise somewhere. New motors have veriable timing to make the best of things.
Moto GP bikes have GPS’s talking to the timing computers and making adjustments for every turn on a given track.
All we can do is pick the cam for the characteristics that we want, and then fine tune it a bit to work with the other components we already have by degreeing.
I think u need to look at your motors week spot and tailor your cam timing to enhance that spot.
If u look at a dyno chart of a motor and see a slight flat spot at say, 5000 rpm u could try adjusting the cam timing to help fill that spot.
I think the best power curve would be 1 that climbs steadily. Not 1 where u feel the cams kick it hard at 1 particular rpm. This is best for a street ride IMO. A drag bike would be dif.
This is a big subject. All u really need to do is look for your motors weakness and figure out witch way to degree it,to help it. If that isn’t enough, then u need a dif component.
Weather that is a dif cam, carb, or pipe.
I hope this is of some help.
76 KZ, frame gusset work,1200CC.Ported by Larry Cavanaugh, 1.5mm.over intakes, Carron Pipe, ZRX12 rear end, and seat,96zx9 front end.
01 CBR600F4i Track bike.
Cobourg, Ont. Can.
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~ (k) / (z)
